Solid oxide fuel cells (SOFC) have demonstrated great potential for future power generation with high efficiency and low emission. Due to relatively high operating temperature and stable electrolyte material, SOFCs can be operated with the hydrocarbon reformate fuel containing higher carbon monoxide and carbon dioxide content than other types of fuel cell systems. Furthermore, the SOFCs can be integrated with a gas turbine to further enhance the overall system efficiency through co-generation.
At present, an SOFC stack generally requires an upstream, separate reforming process when hydrocarbons such as natural gas, gasoline, diesel, jet fuel, and the like, are used as fuel for the SOFCs. External reforming converts hydrocarbons into a mixture containing hydrogen and carbon monoxide, carbon dioxide, etc., which is also known as reformate. The reformate is subsequently fed into the anode side of the SOFC stack and is converted to electric energy through the electro-chemical reaction at the surface of the electrode.
Types of reforming processes include catalytic partial oxidation (CPOX), autothermal reforming (ATR) and steam reforming (SR). Such external reforming processes invariably add volume, cost and operating complexity into the total SOFC power generation system. Moreover, they often consume additional energy in the process of converting hydrocarbons. For example, CPOX and ATR processes require mixing oxidizing gas with hydrocarbons so that a portion of the hydrocarbons is oxidized to generate sufficient heat for the overall catalytic process. External steam reforming is an endothermic process and requires a heat source, which is typically a separate combustor that consumes additional fuel. Alternatively, the thermo energy released from an SOFC stack can be utilized to drive the SR reaction through a costly heat exchanger. Ideally, the hydrocarbon reforming process should be carried out inside the SOFC stack through so-called “internal reforming.”
SOFCs typically operate at above 700° C. which is a suitable temperature for steam reforming. Heat generated through electro-catalytic oxidation over electrodes and ohmic resistance over electrolyte in an SOFC can be utilized to drive the reforming reaction. Therefore, the internal reforming process does not need a costly external device.
There are several challenges currently existing in the internal reforming approach. The first one is a lack of catalytic surface area for sufficient reforming activity. The current practice in internal reforming relies on the heterogeneous reaction occurring on the anode side of an SOFC. The mixture of hydrocarbons and steam react catalytically at the surface of the anode to form hydrogen and carbon monoxide, followed by the electro-catalytic oxidation with oxygen anions. Since the anode in an SOFC usually consists of low surface area cermet material as the result of the type of metal oxide used and high temperature preparation, the catalyst area by anode alone is not sufficient for the reforming need.
The second challenge in internal reforming is the formation of carbonaceous deposit over the surface of the anode, which normally contains nickel. This is the result of dehydrogenation polymerization occurring at elevated temperature. This problem is compounded when liquid hydrocarbon fuels are used. The carbon formation blocks fuel passage to active sites in the anode and results in performance decay of SOFC. New anode materials replacing nickel with ceria and copper are being developed to avoid carbon deposition while promoting direct hydrocarbon oxidation. However, low activity of copper and complexity of fabrication will probably limit the practical application of these anode materials. Another way to reduce carbon formation on an SOFC anode is to add steam into the hydrocarbon fuel to remove carbonaceous deposit through steam reforming. However, since an Ni-ZiO2 based anode is not optimized for steam reforming reaction, a high steam to carbon ratio is needed to make the process effective. Therefore, a method that could significantly improve steam forming efficiency inside an SOFC stack with no carbon deposit on the anode surface is highly desirable.
U.S. Pat. No. 6,051,329 discloses a method of forming an SOFC anode layer with high porosity and added precious metals to improve the catalytic surface area and reactivity for internal reforming. Such a method has intrinsic drawbacks as (a) it substantially increases the cost of an SOFC due to the usage of precious metals and (b) it increases the chance of carbonaceous deposit formation directly over the porous anode thus hinders the fuel mass transfer for effectively electro-oxidation. It is therefore desirable to develop method of internal reforming that will not be costly and will not generate carbon deposit blockage over the anode surface.
Interconnect plates with intermetallic compositions such as NiAl or Ni3Al which can catalyze steam reforming of hydrocarbons were disclosed in U.S. Pat. No. 5,496,655. This method requires fabrication of the bipolar interconnection plate by mixing the nickel and aluminum powders with ceramic filler, followed by high pressure compression and high temperature treatment. Such a method will produce intermetallic bipolar plate with higher surface area usable for internal fuel reforming. There are, however, disadvantages of this method such as (a) a high pressure compression method that requires expensive equipment and added energy cost for the production, (b) a ceramic filler in the intermetallic composite that causes increase of the electric resistance, and (c) severely deactivated internal reforming activity due to the surface carbon deposit since the majority of the catalytic surface is embedded inside of the interconnection plate. It is therefore desirable to develop a new type of interconnection plate with catalytic reforming activity which can be made through low cost production method. It is further desirable to develop the catalyzed interconnection plate with high electric conductivity so that the catalytic function will not be easily affected by carbon formation.